In the VAD method of fabricating lightguide preforms, materials such as SiCl.sub.4, GeCl.sub.4, POCl.sub.3, BBr.sub.3 or the like are fed into an oxy-hydrogen torch and the fine glass soot particles produced by a flame hydrolysis reaction are deposited onto the end surface of a rotating support member as described in U.S. Pat. No. 4,224,046 to Izawa et al. which issued on Dec. 13, 1977. The support member is slowly withdrawn from the torch, as deposition continues, resulting in an elongated, porous soot boule being attached thereto. The boule thus formed is then consolidated into a transparent lightguide preform by heating at an elevated temperature.
Typically, torches used to form the glass soot are unitary fused silica glass structures comprised of a plurality of concentrically aligned cylindrical tubes with means for feeding various gases and reactants into and through the appropriate tubes. Clearly, such torches, once fabricated have fixed dimensions and cannot be readily altered or adjusted which requires that a totally new torch be fabricated at considerable cost.
A torch that overcomes the foregoing problems is described in U.S. patent application Ser. No. 248,906 titled "Universal Torch" filed Mar. 30, 1981 to which a continuing application Ser. No. 486,370 was filed on Apr. 19, 1983. by H. M. Presby and assigned to Bell Telephone Laboratories. That torch is of a reconfigurable construction which can be readily assembled and disassembled to replace and repair parts.
Although such a torch has many advantages over the torches of the type described in the prior art, once in operation the various components become fixed. At times it may be desirable to alter or adjust the location of the discharge end of the inner reactant directing tube to dynamically change the deposition of reactants to controllably alter the refractive index of the resultant lightguide preform. Additionally, it has been found most difficult to fabricate and to maintain such torches in accurate concentric alignment. Lack of such accurate alignment results in nonuniform deposition of reactants on the sootform which can result in poor transmission in fibers drawn therefrom.
Accordingly, there is a need for a reconfigurable torch having accurately spaced concentric tubes wherein the discharge end of the reactant directing tube can be controllably altered during deposition.